This invention relates to screw presses.
A screw press conventionally comprises a press frame, a press slide movable in said frame, a press spindle coupled in screw threaded manner to the press slide for moving the latter on rotation of the spindle, drive means for rotating the spindle, a friction clutch between the drive means and spindle for rotationally coupling the drive means to the spindle on engagement of the clutch, clutch-operating pressure fluid actuated cylinder and piston means for engaging the clutch by application of pressure fluid to the cylinder and piston means; and a pressure fluid supply.
Commonly, the drive means comprise a flywheel, coaxial with the spindle.
In one commonly used construction, the friction clutch comprises an annular clutch disc or plate which can be moved axially by a clutch-operating piston, a clutch disc or plate connected to the spindle, and friction members, all disposed in an internal space enclosed by the flywheel and a clutch cover mounted on the flywheel. Return springs are provided, acting in the opposite direction to the clutch operating piston, to return the annular pressure disc to its initial position for releasing the clutch.
As the friction members become worn in operation, the operating piston has to execute an increasingly long stroke with a corresponding increase in the volume of pressure fluid required for operation to engage the clutch. This has the disadvantageous result that the operating characteristics of the press vary, as the friction members wear.
To overcome this disadvantage, screw presses have been provided with means to limit the stroke of the operating piston for disengaging the clutch and also for engaging the clutch.
Mechanical stroke-limiting means require constant monitoring of the friction member wear and corresponding manual adjustment of the stroke-limiting means.
German Pat. No. 3503917 describes an automatic hydraulically operating stroke-limiting means. In this, a metering unit comprises a metering piston which can move, with a limited stroke, in a metering cylinder. One end of the metering cylinder is connected to the pressure medium supply line, and the other end is connected to the cylinder of the clutch operating piston. A bridging line is provided in parallel with the metering cylinder and contains a non-return valve. To engage the clutch, pressure medium is applied to the metering cylinder, causing the metering piston to advance and expel, into the operating cylinder, pressure medium already present in the metering cylinder; the non-return valve remains closed while the metering piston advances, so that the clutch operating cylinder is not directly connected to the pressure medium supply. When the metering piston has advanced to its end position, a further pressure increase causes the non-return valve to open, so that the operating piston is loaded with the full pressure of the pressure medium supply. To release the clutch, the pressure medium is released and the non-return valve closes. The clutch pressure disc is lifted away from the clutch friction members by springs, acting on the pressure disc through pull rods, and carries the clutch operating piston with it, the volume between the clutch operating piston and the metering piston remaining full of pressure medium. The stroke of the operating piston is limited, by the restriction of the stroke of the metering piston in the metering cylinder. It will be understood, that the non-return valve operates to top up the volume of fluid in the clutch operating cylinder, as the clutch stroke increases with wear.
This apparatus however has the disadvantage that, if some form of failure prevents the metering piston from advancing, pressure will build up behind it and cause the non-return valve to open, thereby applying full fluid pressure to the clutch operating piston even though the metering piston has not reached its advanced end position. If then the pressure medium is released, the non-return valve will close, trapping pressure medium between the metering piston and clutch operating piston. Because the metering piston cannot move, or if it can move in its reverse direction, can move only by a reduced amount, the pressure medium cannot be fully discharged from the clutch operating cylinder into the metering cylinder, the friction clutch is not released or not fully released, and the screw press can be more or less severely damaged.
An object of the present invention is to provide stroke-limiting means for the clutch of a screw press, such as to provide automatic compensation for wear of the friction clutch, while ensuring proper release of the clutch even if the stroke-limiting apparatus fails.